The Role of Pleistocene Refugia and Rivers in Shaping Gorilla Genetic Diversity in Central Africa
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The role of Pleistocene refugia and rivers in shaping gorilla genetic diversity in central Africa Nicola M. Anthony*†‡, Mireille Johnson-Bawe§, Kathryn Jeffery†¶, Stephen L. Clifford§, Kate A. Abernethy§¶, Caroline E. Tutin§¶, Sally A. Lahmʈ, Lee J. T. White**, John F. Utley*, E. Jean Wickings§, and Michael W. Bruford† *Department of Biological Sciences, University of New Orleans, New Orleans, LA 70148; †School of Biosciences, Cardiff University, Cardiff CF10 2TL, United Kingdom; §Centre International de Recherches Me´ dicales de Franceville, B.P. 769, Franceville, Gabon; ¶School of Biological and Environmental Sciences, University of Stirling, Stirling FK9 4LA, United Kingdom; ʈInstitut de Recherche en Ecologie Tropicale, B.P. 180, Makokou, Gabon; and **Wildlife Conservation Society, Bronx Zoo, 185th Street and Southern Boulevard, Bronx, NY 10460 Edited by Jeffrey Rogers, Southwest Foundation for Biomedical Research, San Antonio, TX, and accepted by the Editorial Board November 6, 2007 (received for review May 23, 2007) The role of Pleistocene forest refugia and rivers in the evolutionary refugia in geographical speciation (29, 30). However, it could diversification of tropical biota has been the subject of consider- also be argued that the recent time frame of Pleistocene events able debate. A range-wide analysis of gorilla mitochondrial and requires a population-genetic rather than a species-level ap- nuclear variation was used to test the potential role of both refugia proach. Another criticism centers on the real difficulties of and rivers in shaping genetic diversity in current populations. pinpointing the precise location of putative refugia (4) and Results reveal strong patterns of regional differentiation that are discriminating between competing modes of diversification (5). consistent with refugial hypotheses for central Africa. Four major Molecular data can, however, be used to infer signatures of past mitochondrial haplogroups are evident with the greatest diver- history and, in so doing, test several important predictions of gence between eastern (A, B) and western (C, D) gorillas. Coales- Pleistocene hypotheses (3, 7, 31): (i) allopatric fragmentation is cent simulations reject a model of recent east–west separation evident and coincident with the possible location of hypothesized during the last glacial maximum but are consistent with a diver- refugia; (ii) signature(s) of demographic expansion within refu- gence time within the Pleistocene. Microsatellite data also support gial populations are apparent; and (iii) haplotype exchange is a similar regional pattern of population genetic structure. Signa- coincident with boundaries of adjacent refugia. tures of demographic expansion were detected in eastern lowland In contrast, the riverine barrier hypothesis argues that tropical (B) and Gabon/Congo (D3) mitochondrial haplogroups and are rivers limit species distributions (32, 33) and shape intraspecific consistent with a history of postglacial expansion from formerly patterns of diversification (34, 35). To date, support for this isolated refugia. Although most mitochondrial haplogroups are hypothesis is equivocal and depends heavily on the ecology and regionally defined, limited admixture is evident between neigh- dispersal abilities of individual taxa. In central Africa, the Sanaga boring haplogroups. Mantel tests reveal a significant isolation-by- River is an important biogeographic boundary for several distance effect among western lowland gorilla populations. How- groups, including some primates and forest duikers (25, 36). For ever, mitochondrial genetic distances also correlate with the chimpanzee subspecies, however, this proposed barrier may be distance required to circumnavigate intervening rivers, indicating incomplete (37). Other studies have also reported that rivers a possible role for rivers in partitioning gorilla genetic diversity. influence genetic differentiation between mandrill (21) and Comparative data are needed to evaluate the importance of both mechanisms of vicariance in other African rainforest taxa. bonobo (38) populations. Where sampling is of sufficient inten- sity, molecular data can be used to test predictions of the riverine control region ͉ mitochondrial ͉ phylogeography ͉ refugium barrier hypothesis, namely, that genetic variation is structured across different river banks, and that genetic differentia- tion decreases from the mouth to the headwaters of a given echanisms underlying evolutionary diversification in trop- watercourse. Mical forests have intrigued biologists for more than a Gorillas are good candidates for testing geographically explicit century. Numerous hypotheses have been proposed (1–2), of hypotheses of vicariance because of their strong association with which the Pleistocene forest refugium and riverine barrier closed canopy forest and their restricted ability to traverse hypotheses have provoked considerable interest and controversy savannah–forest mosaic habitats (39). Gorillas may also be good (3–7). Whereas most studies have focused on the Amazon and models for testing the isolating effects of rivers because the Australian wet tropics (5, 8–10), data on central African rain- taxonomic boundaries of many primates appear coincident with forest taxa remain relatively sparse. According to Pleistocene refuge theory, forest fragmentation major river courses (33, 36). By drawing on data from a during glacial maxima led to the isolation and subsequent range-wide analysis of gorilla populations, we aim to uncover the diversification of forest-associated taxa (3). During periods of relative importance of rainforest refugia and rivers in shaping climate amelioration and population expansion, zones of sec- gorilla population structure and, in so doing, provide insight into ondary contact may have also formed between neighboring refugial populations. Although palynological and biogeograph- Author contributions: K.A.A., L.J.T.W., E.J.W., and M.W.B. designed research; N.M.A., ical data have been used to infer forest refugia (11–16), their M.J.-B., and S.L.C. performed research; K.J., K.A.A., C.E.T., S.A.L., L.J.TW., and J.F.U. con- precise location and role in Pleistocene diversification is con- tributed new reagents/analytic tools; N.M.A., J.F.U., and M.W.B. analyzed data; and N.M.A. troversial. Several molecular studies suggest that refugia may wrote the paper. have played an important role in structuring montane birds The authors declare no conflict of interest. (17–19), primates (20–22), and trees (23). However, other forest This article is a PNAS Direct Submission. J.R. is a guest editor invited by the Editorial Board. species such as chimpanzees (24–26) and elephants (27, 28) show Data deposition: The sequences reported in this paper have been deposited in the GenBank relatively weak regional genetic structure, suggesting that wide- database (accession nos. EU305296–EU305397). ranging and/or savannah-tolerant species may be poor indicators ‡To whom correspondence should be addressed. E-mail: [email protected]. of range changes in tropical forest cover. One criticism of the This article contains supporting information online at www.pnas.org/cgi/content/full/ Pleistocene refuge hypothesis argues that species divergence 0704816105/DC1. times often predate the Pleistocene, undermining the role of © 2007 by The National Academy of Sciences of the USA 20432–20436 ͉ PNAS ͉ December 18, 2007 ͉ vol. 104 ͉ no. 51 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0704816105 12 9 28 Haplogroup A 18 Haplogroup B Haplogroup C1 Haplogroup C2 Haplogroup D1 Fig. 2. Geographic distribution of major mitochondrial haplogroups across Haplogroup D2 central Africa. Important rivers are also indicated with an arrow. CR, Cross Haplogroup D3 5 River; SA, Sanaga River; OG, Ogooue´ River; IV, Ivindo River; SG, Sangha River; 6 UB, Ubangui River; CG, Congo River. The locations of two published museum samples (22, 57) are indicated by ‘‘?.’’ 7 are in Lobe´ke´, southeastern Cameroon (site 4), and the Monts de Cristal in northwestern Gabon (site 6). Molecular diversity is greatest in western gorilla haplogroups Fig. 1. Minimum spanning tree of mitochondrial HV1 haplotypes identified C1/2, D1, and the heavily admixed Ivindo population (SI Table in all three major gorilla subspecies. The mutational steps separating haplo- types are indicated by using cross bars or by number. Major mitochondrial 1). When gorilla populations were pooled into eastern or western haplogroups (A–D) and their respective subdivisions (C1–2, D1–3) are based on groups, molecular diversity indices (40) and (41) were almost haplogroups recovered in the phylogeny (see SI Figs. 4 and 5). 2-fold higher for western gorillas ( ϭ 0.047, ϭ 0.058) than for eastern gorillas ( ϭ 0.029, ϭ 0.038). Fu’s F and Tajima’s D statistic are significantly negative for the eastern lowland gorilla historical processes that underlie the evolution of tropical forest haplogroup B and western gorilla haplogroup D3. Only Tajima’s biota. D was significant for the geographically widespread haplogroup C. Mismatch distribution profiles provide a strong unimodal Results pattern for haplogroups A, B, and D3 (data not shown). In The minimum spanning network (Fig. 1) illustrates the deep keeping with earlier recommendations (7) and the acknowl- divergence between eastern and western gorillas and the high edged upward bias in estimates of the growth parameter g (42), levels of substructuring present in western gorillas relative to only haplogroups B and D3 demonstrated